How can I use a Lambda function as an AWS AppSync subscription resolver

Question

I am working on an AWS AppSync driven app. I have the following dynamo object...

type Message
    @model
    @key(name: "bySender", fields: ["senderId"])
    @key(name: "byDate", fields: ["channelId", "createdAt"], queryField: "getMessagesByDate")
    @auth(
        rules: [
            { allow: groups, groups: ["user"], operations: [read, create, update] }
        ]
    ) {
    id: ID!
    channelId: ID!
    senderId: ID!
    channel: Channel @connection(fields: ["channelId"])
    sender: User @connection(fields: ["senderId"])
    createdAt: AWSDateTime!
    text: String
    fileKey: String
    hasBeenReadByIds: [ID!]
}

That I create messages with the following mutation...

createNewMessage(id: ID!, channelId: ID!, senderId: ID!, text: String, fileKey: String): Message

I have a subscription that I use to listen for messages created with createNewMessage...

onCreateMessageForMe: Message @aws_subscribe(mutations: ["createNewMessage"])

The subscription uses a javascript lambda as a resolver. But when the lambda gets called, event.arguments is {}.

exports.handler = async (event) => {
    console.log("EVENT  ---- ", event);
}    

EVENT  ----  {
  arguments: {},
  identity: {
    claims: {...

How to I get the attributes (id, senderId, channelId, text...) of the newly created message from the event?